The reason for your Dell wireless printer to show offline is because of its short hibernation period either set by your manufacturer or maybe yourself. In order to resolve the issue, I will discuss a few troubleshooting techniques, follow them carefully:
Change Hibernation/Sleep mode settings
If your printer is getting into sleep or hibernation mode too early and too much, then chances are it will be offline. So go to your printer's settings and change the time period of the hibernation mode to the maximum which will help your printer to not go to sleep mode due to a small period of ineffectiveness.
Update your Drivers
Make sure you are using the latest available driver for your printer as these drivers are directly responsible for all the functionalities in your router. Make sure to update these drivers from time-to-time.
Restart your Printer
If the issue prevails even after making necessary changes, then try rebooting your printer setup along with the router, printer, and the Mac. Shut it down for few minutes and then restart the setup to see if the issue is resolved.
Reconfigure your printer with Mac
If the issues continue to prevail and do not gets fixed by following the above-mentioned steps, then reconfigure your Dell printer with your Mac PC and make sure it is a default printer for the Mac PC.
Hopefully, these steps helped you in resolving your printer issues. In case you have any further doubts or queries, feel free to reply back and I will be glad to help.